1. Field of the Invention
The described invention is in the field of devices and methods of use for correction of spinal deformities or injuries. More particularly, the described devices and methods are directed to a correction of spondylolisthesis by spinal reduction, or repositioning of the spine to a more normal configuration.
2. Description of Related Art
Spondylolisthesis is a forward movement or slippage of a lower lumbar vertebra on the vertebra below it, or on the sacrum. When a spinal fusion is necessary or recommended, a physician must decide whether to do a reduction, or to fuse the vertebra in the forward position. The latter is often done because of the danger of nerve or tissue damage associated with reduction. Early methods of spinal reduction included cable systems that were attached to a vertebra and connected to an external system of traction weights.
An apparatus for spinal reduction is described in U.S. Pat. No. 4,611,581 to Steffee. The apparatus includes a spinal plate or a pair of plates that may be screwed to several adjacent vertebrae. A bone screw in the displaced vertebra is attached to, or includes a threaded rod, which then passes through an opening in the plate. By means of tightening a nut on the threaded rod, the misalignment may be reduced.
Another system is described in U.S. Pat. No. 5,782,831, to Sherman et al. In the described system, a cable is attached, either directly or indirectly to a bone screw in a misaligned vertebra. The cable may then be attached to an anchor on a spinal rod running along the spinal column. A cable tensioning device is then used to apply tension to the displaced vertebra to pull it into the desired position.
Because of the natural curvature of the spine, and the shape of individual vertebrae, a misaligned vertebra typically needs to be pulled in a direction that is not parallel to the long axis of a bone screw in that vertebra. It is, in fact, the need to apply a non-parallel force that may be a cause of damage to nerve or soft tissue, or even failure of the bone. There is a need therefore, for a system for spinal reduction in which a force can be applied to a misaligned vertebra in the desired direction when that direction is not parallel to the long axis of a bone screw in that vertebra.